1. Field of the Invention
Embodiments of the present invention generally relate to giant magnetoresistive (GMR) sensors. More particularly, the invention relates to current-perpendicular-to the-plane (CPP) magnetoresistive sensors.
2. Description of the Related Art
Magnetic read heads in modern magnetic disk drives operate on the basis of the tunneling magnetoresistive (TMR) effect in which a component of the read element resistance varies as the cosine of the angle between the magnetization in free and reference magnetic layers which sandwich an insulating tunnel barrier layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (i.e., the signal field) causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance across the tunnel barrier in the TMR and a corresponding change in the sensing electrical current or voltage. As read head sizes scale down to accommodate increasing areal storage densities, the device resistance of TMR read heads is projected to increase beyond levels that are easily accommodated using standard detection electronics.
A GMR sensor is an alternative to a TMR read head. A GMR read head has a resistance that varies according to the angle between the free and reference magnetic layers. Further, a GMR read head uses a sense CPP of these magnetic layers. CPP-GMR sensors primarily differ in structure from TMR read heads in that the high resistance tunnel barrier layer is replaced by a low resistance metallic spacer. Accordingly, the resistance of a CPP-GMR sensor is primarily determined by stack structure of the free layer, the reference layer, and the low resistance metallic space. Rather than spin-dependent tunneling of electrons across a barrier layer as used in a TMR sensor, the CPP-GMR sensor uses spin-dependent scattering of the conduction electrons at both the interface between the magnetic and spacer layers as well as in the magnetic layers themselves. For a given cross-section area, the device resistance of a CPP-GMR sensor will be 10-20 times smaller than for a TMR sensor.
Because of the much lower device resistance, CPP-GMR sensors operate at much higher sense current densities if the bias voltage applied is comparable to that used in TMR sensors (e.g., 100 mV). At these higher current densities, the output signal and signal/noise ratio for a CPP-GMR sensor is limited by spin-torque effects, which originate from the torque induced on either the free or reference magnetic layers by the spin-polarized electron current density applied during operation. If too large, the spin-torque can introduce oscillatory instability of the magnetization in either the free or reference layers. Accordingly, reducing the susceptibility of a CPP-GMR read sensor to spin-torque induced instability improves its performance in magnetic recording applications.
Moreover, during processing of a read head, a CPP-GMR sensor may be subjected to lapping or chemical mechanical polishing/planarization (CMP) during which exposed layers may begin to corrode. The metallic spacer layer may be particularly susceptible to oxidizing during this process. For example, common spacer layer materials are known to easily corrode or tarnish. During a mechanical lapping process to form the air bearing surface (ABS), these materials may oxidize and hamper the electrical current flowing between the different layers of the CPP-GMR sensor.